Without calculating the exact angle

MATHEMATICS · Subjective · AP · AP Precalculus · Functions with Parameters, Vectors, and Matrices · Functions with Parameters, Vectors, and Matrices

Problem

Without calculating the exact angle, determine whether the angle between the vectors

\mathbf{u}=(2,-3,1)

and

\mathbf{v}=(-4,6,-2)

is acute, obtuse or right. Give a brief reason for your answer.